Integrated circuit manufacturing and packaging processes commonly employ electroplating to deposit metal layers on conductive parts. A lead frame, for example, made of a copper alloy may be plated with a layer of solder to facilitate soldering or wire bonding to the lead frame during an integrated circuit packaging process.
A reliable electroplating process for a lead frame or another electronic component generally requires several preparatory processes before a plating operation deposits metal. An activation process, for example, is commonly used to prepare the surface of the work piece (e.g., a lead frame) for electroplating. For the activation process, the work piece is connected to a power supply and placed in a solution containing an activation chemical such as an acid. An electrolysis process can then clean and otherwise prepare the surface of the work piece to accept a uniform electroplated layer.
The actual plating operation conventionally places the prepared work piece into a solution and connects the work piece to the negative pole of a DC power supply. As a result, the work piece acts as a cathode in the solution. An electrode connected to the positive pole of the power supply is also placed in the solution and acts as an anode. The solution and/or the anode contain the metal or metals (as ion or neutral atoms) that will be plated onto the work piece when the power supply drives a current from the anode to the cathode through the solution. The current includes positive metal ions in the solution that flow to the work piece where the metal ions pick up electrons and become neutral atoms attached to the surface of the work piece.
The quantity of metal deposited on the work piece during the plating operation depends on the amount of current through the work piece. As a result, the electroplating operation generally deposits more metal in areas where current concentrations are higher. At sharp high points on rough surfaces, the high current concentration can cause growth of whiskers of the plated metal. Such whiskers are undesirable on lead frames since the whiskers can cause electrical shorting and/or visible defects.
A controlled electroplating of a lead frame having a smooth surface can avoid unacceptable whisker formation. However, when the plating process is interrupted, for example, by a mechanical breakdown or for any other reasons, a batch of lead frames that was in the electroplating equipment during the process interruption is found to have a higher than normal incidence of whisker formation after completion of the plating process. In particular, leaving work pieces in an activation solution during a process interruption can roughen the surfaces of work pieces, which leads to whisker formation. A stoppage of 30 minutes, for example, during plating of a batch of lead frames can significantly reduce the yield of good parts.
In view of these difficulties, processes and structures that provide a high yield of good quality parts even after an interruption of a plating process are sought.
In accordance with an aspect of the invention, a device such as a lead frame made of an alloy receives a protective strike of a metal having a high purity, preferably more than 99.9% pure. Pure tin (Sn), for example, can be plated on a copper alloy lead frame during an activation process. The lead frame can subsequently be plated with solder or another metal using conventional electroplating techniques. In event of an interruption of the electroplating process, the strike of pure metal interacts evenly with activation chemicals to preserve the smooth surfaces of the lead frame. Accordingly, formation of whiskers is avoided when the plating process resumes.
One specific embodiment of the invention is an electroplating process for a metal alloy work piece such a lead frame made predominantly of copper. The process includes placing the work piece in an activation solution containing an activation chemical and a plating chemical and running a current through the work piece and the activation solution to deposit a layer of a pure metal (e.g., a tin strike) on a surface of the alloy during an activation operation. In the event of an interruption of the electroplating process, the activation solution etches the pure metal causing less surface roughening than would etching of the alloy. After the activation operation, the process electroplates a solder (e.g., Snxe2x80x94Pb) layer over the pure metal on the surface of the alloy.
The pure metal layer, which serves to protect the work piece during process interruptions, can be much thinner than the solder layer, so that the pure metal layer minimally affects soldering, electrical, and other characteristics of the plated work piece. The solder layer may, for example, be five or more times the thickness of the pure metal layer. Specifically, the layer of pure metal may have a thickness less than about 2.0 xcexcm, while the solder layer has a thickness that is more than about 10 xcexcm.
Another specific embodiment of the invention is a lead frame including a body made of an alloy of a metal such as copper, a pure metal plating on the alloy, and a solder plating on the pure metal plating. The pure metal plating can be a tin strike that provides a base when the solder plating is made of tin and lead. The solder plating is generally much thicker than the pure metal layer to provide the lead frame with good soldering characteristics. The solder layer can be five or more times thicker than the pure metal plating. In one specific case, the solder plating has a thickness that is more than about 10 xcexcm, while the pure metal plating has a thickness less than about 2.0 xcexcm.